This invention pertains to electrical stimulation leads in general, and to myocardial pacing leads in particular.
Electrical stimulation of the body for medical purposes is well known in the prior art. An example of a device for this purpose is the well known cardiac pacemaker. In the pacemaker context, as well as other body stimulation contexts, the stimulation energy is delivered to the desired body site by an electrode carrying lead.
Interactions between the lead and body can vitiate the desired effects of the stimulation. For example, biologic reactions encourage fibrosis. In the pacemaking context, fibrosis is believed to be a major factor in the increase in chronic stimulation threshold that is usually experienced. Also, trauma results in inflammation of the tissue to be stimulated. Such inflammation may alter the response of the tissue to the stimulation energy, both acutely and chronically.
Other interactions between the lead and body, while not directly affecting the response of the tissue to the stimulation energy, can result in the occurrence of undersirable events. In some circumstances where electrical body stimulation is indicated, the body portion to be stimulated is irritable. The placement of a lead may compound this irritability. For example, placement of a pacemaking lead may include a cardiac arrhythmia. The presence of the lead may also promote thrombus formation.
The interactions noted above have long been recognized and efforts made to ameliorate their consequences. For example, the lead may be configured to reduce mechanical trauma and the response of the irritable tissue during lead placement. Materials may be selected for the lead body and electrodes to minimize fibrosis. However, lead configuration must take into account other factors such as the efficiency of the delivery of the stimulation energy, the ease of placement, maintenance of the desired electrode position, and the reliability of the lead over extended periods of time. An accomodation of these interests has resulted in leads whose configuration necessarily results in undesirable interactions between the lead and body.
Undesirable interactions between lead and body are a particular problem in the context of a myocardial lead employing a penetrating electrode of the sort employed by the present invention. Unlike the electrodes of most endocardial leads, penetrating myocardial electrodes are intended to function by insertion into the tissue, rather than by placement against the tissue. As such, the basic mode of operation of these electrodes requires some small amount of tissue damage to accompany attachment of the electrode. The greater the reaction of the heart tissue to this injury, the more likely it is that subsequent problems of increased threshold, fibrosis or irritability will rise.
It is also known that a systemic treatment with steroids results in acute reduction in the stimulation threshold. In particular, systemic use of glucocorticosteroids has been used to treat chronic exit block, a condition in which the heart tissue's reaction to the pacing lead can result in a chronic rise of the stimulation threshold above the output level of the implanted pacemaker. However, long term systemic use of such steroids produces undesirable side effects.
In particular, copending commonly assigned Application Ser. No. 476,436 referenced above discloses an endocardial pacing lead adapted to dispense a glucocorticosteroid adjacent the stimulation site.